Photolithography is used to produce electronic optical or mechanical microstructures, or microstructures combining electronic and/or optical and/or mechanical functions. It consists in irradiating, with photon radiation, through a mask that defines the desired pattern, a photosensitive resin or photoresist layer deposited on a planar substrate (for example a silicon wafer). The chemical development that follows the irradiation reveals the desired patterns in the resist. The resist pattern thus etched may serve for several usages, the most common being the etching of an underlying layer (whether insulating or conducting or semiconducting) so as to define therein a pattern identical to that of the resist.
It is sought to obtain extremely small and precise patterns and to align etched patterns very precisely in multiple superposed layers. Typically, the critical dimension of the desired patterns is nowadays a fraction of a micron, or even a tenth of a micron and below.
Attempts have been made to use photolithographic processes not using light but electron or ion bombardment. These processes are more complex than photolithography processes using photons at various wavelengths (visible, ultraviolet, X-ray). Sticking to optical photolithography, it is the reduction in wavelength that allows the critical dimension of the features to be reduced. Ultraviolet photolithography (at wavelengths down to 193 nanometers) has become commonplace.
It is endeavored at the present time to go well below these wavelengths and to work in extreme ultraviolet (EUV) at wavelengths between 10 and 14 nanometers which are in practice soft X-ray wavelengths. The objective is to obtain a very high resolution, while still maintaining a low numerical aperture and a sufficient depth of field (greater than 1 micrometer).
At such wavelengths, one particular aspect of the photolithography process is that the resist exposure mask operates in reflection and not in transmission: the extreme ultraviolet light is projected onto the mask by a source; the mask comprises absorbent zones and reflecting zones; in the reflecting zones, the mask reflects the light onto the resist to be exposed, impressing its image thereon. The path of the light between the mask and the resist to be exposed passes via other reflectors, the geometries of which are designed so as to project a reduced image of the mask and not a full-size image. The image reduction makes it possible to etch smaller patterns on the exposed resist than those etched on the mask.
The mask itself is fabricated by photoetching a resist, this time in transmission, as will be explained later, and with a longer wavelength, permitted by the fact that the features are larger.
Typically, a reflection mask is made up of a planar substrate covered with a continuous reflecting structure, in practice a Bragg mirror structure, covered with an absorbent layer etched in the desired masking pattern.
The mirror must also be as reflective as possible at the working wavelength designed for the use of the mask. The absorbent layer must also be as absorbent as possible at this wavelength and must be deposited without causing deterioration of the reflecting structure, which notably implies deposition at not too high a temperature (below 150° C.). It must also be able to be etched without damaging the reflecting structure and in general a buffer layer is provided between the absorbent layer and the mirror. The height of the stack comprising the buffer layer and the absorbent layer must be as small as possible so as to minimize the shadowing effects when the incidence of the radiation is not perfectly normal to the surface of the mask.